The present invention relates broadly to an electromagnetic interference (EMI) shielding or grounding panel assembly including a porous, electrically-conductive shielding media and an electrically-conductive frame for supporting the media, and more particularly to such an assembly which is adapted to cover a corresponding ventilation opening in an electronics housing or other enclosure enclosures and which accommodates the flow of cooling air into the enclosure while maintaining electrical continuity and EMI shielding effectiveness across the opening.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices. xe2x80x9cEMIxe2x80x9d is used herein interchangeably with the term xe2x80x9cradio frequency interferencexe2x80x9d (xe2x80x9cRFIxe2x80x9d).
For attenuating EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is interposed between the source and the other devices, and most often is configured as an electrically conductive and grounded housing or other enclosure, such as a room, which surrounds the EMI generating circuitry of the source device. However, when such circuitry is contained within the confined space of an enclosure, it often is necessary to provide a cooling or ventilation means to dissipate the heat which is ohmicly or otherwise generated by the circuitry. Most enclosures therefore are formed with one or more air intake and/or exhaust openings or ports for natural or forced convective circulation of air between the interior of the enclosure and the ambient environment.
Left uncovered, such openings would represent a discontinuity in the surface and ground conductivity of the enclosure, with the result of a decrease in the EMI shielding effectiveness of the enclosure. Accordingly, shielded vent panels have been proposed for covering the openings in an manner which allows for the ventilation of the enclosure while electrical continuity, i.e., grounding, across the vent opening is maintained. In basic construction, such vent panels, which are sized to span the corresponding opening in the enclosure, conventionally are formed as including a sheet of a porous, electrically-conductive shielding media, and an electrically-conductive frame member configured to support the media as extending about the outer periphery thereof. The media, which may be an expanded metal mesh or, alternatively, a honeycombed-structured or other cellular structured metal foil, is received in or is otherwise attached to the frame, which typically is provided as an extruded aluminum or other metal profile. The frame, in turn, may be fastened to the enclosure over the opening thereof with screws or the like, with a compressible, electrically-conductive seal or gasket optionally provided for improved electrical contact between the frame and the enclosure.
However, for electrical continuity to be maintained across the opening, good electrical contact must be provided not only as between the frame and the enclosure, but also as between the media and the frame. In this regard, conventional panels may employ a C-shaped frame channel including a V-shaped or other projection or edge integrally formed within one of the sides of the channel. With the periphery of the media being received within the channel, the sides thereof are compressed to cause the projection or edge to penetrate into the media and thereby establish good electrical contact. Vent panels of such type are marketed commercially by the Chomerics Division of Parker-Hannifin Corp. (Woburn, Mass. under the tradenames xe2x80x9cCho-Cell(trademark),xe2x80x9d xe2x80x9cShield Cell(copyright),xe2x80x9d and xe2x80x9cOmni Cell(copyright).xe2x80x9d Alternatively, the media may be fit into the frame and then bonded thereto using a conventional joining technique such as resistance welding, brazing, soldering, or the like.
Another method of attaching the media to the frame involves using the gasket typically provided between the frame and the enclosure to hold the filter media in place. As is described in commonly-assigned U.S. Pat. No. 5,032,689, the frame in such method may be integrally-formed as including a ridge over which the gasket is press-fitted for its retention about the periphery of the frame. The gasket, in turn, may be configured to overlap the media for securing the media to the frame.
U.S. Pat. No. 3,580,981 discloses another shielding vent panel wherein an electrically conductive textile is positioned about the periphery of the media for contact between the media and the frame which may have an L-shaped or generally Z-shaped profile. To assure good conductivity, the frame, media, and textile are covered with an electrically-conductive coating or plating such as by immersion in a molten tin or metal bath.
U.S. Pat. No. 4,616,101 discloses a vent including a grating which is arranged in a rectangular metal frame having an L-shaped cross-section that is bent into a U-shape. The grating is clamped between a flange of the frame and supporting feet of a mounting spring. The supporting feet are stamped into the mounting spring, which itself is provided as a strip of metal which is bent rectangularly four times, and are bent into a xe2x80x9cV-shape.xe2x80x9d
Other vents and materials therefor are described in U.S. Pat. Nos. 3,546,359; 3,553,343; 3,584,134; 3,821,463; 4,249,033; 4,616,101; 5,007,946; 5,401,914; 5,895,885; and 5,910,639, JP 8064988, and WO 97/32459.
In view of the proliferation of electronic devices, it is to be expected that continued improvements in EMI shielded vent panels would be well-received by industry, and particularly by the designers of enclosures for personal computers, file servers, telecommunication equipment, and similar systems which now operate at frequencies of 500 MHz or more. Indeed, as the clock speeds of electronic devices continue to increase with the attendant generation of higher frequency EMI radiation and greater heat output, enclosure designers are faced with the seemingly competing requirements of providing both adequate ventilation and effective EMI shielding. In such applications, a honeycomb shielding media, such as is disclosed in U.S. Pat. Nos. 3,821,463; 5,895,885; 5,910,639, would be considered preferred over other media as known to provide effective EMI shielding at higher frequencies with less restriction to air flow. Such materials, however, are relatively expensive and heretofore were believed cost-prohibitive in high volume applications such as personal computers. A preferred vent construction therefore would be economical to manufacture, but also would exhibit both reliable EMI shielding performance and good ventilation in high frequency applications.
The present invention is directed to an EMI shielded vent construction including an electrically-conductive frame member having a generally U- or C-shaped profile including end wall portions and a pair of side wall portions integral with the end wall portion, and an electrically-conductive, porous shielding media member, which is supported by the frame member. At least one of the side wall portions is provided as having a plurality of spaced-apart, finger-like tabs or projections defined therein. The tabs, which are oriented to depend downwardly from the side wall portion for projection into the media, both retain the media within the frame and provide electrical contact with the media. In a preferred construction, the projection of the tabs into the media effects the compression thereof to provide a greater surface area for electrical contact with the frame.
Advantageously, the provision of the economical tabs obviates the need to integrally form a contact projection within the frame member, and thus allows the frame member to be constructed of a stamped metal sheet or roll-formed channel rather than of a more costly extrusion profile. The use of a stamped sheet or a roll formed channel, moreover, allows the frame to be formed by drawing, stamping, folding, or other cold working operation, and thereby eliminates the need for welds at the frame member corners. For reasons of cost and performance, the frame typically will be formed of aluminum or another metal, with the media being formed of one or more layers of a cellular aluminum or other metal foil honeycomb.
In one disclosed embodiment, the tabs are formed to define the periphery of the corresponding side wall portion of the frame. Such construction advantageously facilitates the manufacture of the frame from a polygonal stamping or other generally-planar metal sheet having an outer margin defined by the tabs and an inner margin which may be generally linear. With the corners of the sheet being notched, the sheet is folded intermediate its inner and outer margins into an L-shaped configuration wherein the tabs are oriented generally vertically relative to the opposing side wall. The media then is inserted into the partially-formed frame, and the tabs are folded downwardly, such as in a press or the like, into the media. Optionally, one or more tabs, each of which may be sized the same or differently than the remaing tabs, may be left upstanding for use in attaching the frame to the housing of the enclosure.
In another disclosed embodiment, the tabs are perforated or otherwise formed within the corresponding side wall portion of the frame. Such construction extends the advantages of the present invention to the use of roll-formed channels, which are described more fully in co-pending application U.S. Ser. No. 09/243,778, as a substitute for more costly extruded weldments.
The present invention, accordingly, comprises the EMI shielded vent panel possessing the combination of elements and construction which are exemplified in the detailed disclosure to follow. Advantages of the present invention include a vent panel which exhibits reliable EMI shielding and air flow characteristics. Additional advantages include a panel construction which is economical in allowing the use of a metal stamping or roll formed frame, and which ensures good electrical contact between the shielding media and the frame without the need for additional gaskets, metal plating, or other components or operations. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.